what does “STIFF” mean?

Everyone in the road community knows the value of carbon for reducing ‘buzz’.

Define “knows”.

splay too

By that I presume you mean the fork flex TJ mentioned earlier?

Interesting. In the context of frame vibration I would have referred to that as damping, but you live and learn.

OK brant – no detectable flex. I was putting a large torsional load on the stem and I could detect no flex. The unloaded side of the bar had no visible movement in relation to the bike frame The bars were flexing nicely – about 10mm on the loaded side but the unloaded side did not move in any detectable way relative to the bike frame.

Define “knows”.

It’s widely accepted that carbon frames absorb a lot of road buzz. My own experience backs this up to an extent. Are you going to tell me (and the roadie community) that this is all nonsense and we’re being duped by carbon frame marketing?

quite easy to engineer soemthing to bend in one or mroe directions only.

take a sheet of paper, its quite easy to bend to make a tube,

now try this
bending it in one of the other two dimensions to make a circle? its impossible.

This engineering stiffness in one dimension is quite straightforeward.

so apply this to a bikes chainstays, you want them as stif as possible horisontaly, so make them as square as possible (BB shell, stays, hub axel is your square) now brace it (only possible arround the BB).

You could ride a bike like this, but it would probably be too flexy (probably wouldnt work at all in fact).

So now you add some seat stays, make them as long as possible to make them springy (GT tripple triangle, dropped top tubes, jones, retrotec etc), and make them just stiff enough to keep everything working.

Torsional stiffness is usualy dealt with by the dropouts, a 12mm axel clamped tightly will hold the axel and dropouts in a fixed allignment, stoping one moving independantly of the other, thus keeping everything stiff.

The trouble with that analogy, tinas, is that a bicycle frame is rather more like a vertical piece of paper than a horizontal one. Any conventional frame design is far, far more stiff in a vertical direction than laterally, since for the rear triangle to flex vertically you have to compress the stays along their length ( 😆 at the idea that GT stays make the bike more flexible as they’re longer, given they’re welded onto the seat tube).

molgrips – yes you are all being duped! Deflections are so small that even though the hysteresis is a lot higher the carbon frame can’t possibly be having the magic effect which is attributed to it.

Booger, didnt realise I was a complete numpty! Surely the thing about Aluminum being a dead material is right? Or does it feel dead in relation to steel because of the diameter of tubing and wall thickness?

Certainly ally has a different ‘feel’ to steel or Ti, regardless of whatever metallurgy and engineering, (and Brant ;0)), might say. Some years back I had a little hardtail, a Cannondale M800 Beast Of The East, with XT, ’97 orange Bombers, and a pretty stiff wheelset consisting of Mavic D321 rims on Hope XC hubs. Fun little bike, 14″ BB height, rode it around Chamonix. Then got a Handjob frame and transferred everything but the vee-brakes to it and rode a 17 mile loop on the Marlborough Downs I’d ridden the week before. The difference was startling. Previously I’d got to the end feeling absolutely hammered, but with the steel frame I felt much less tired, like I could do the same loop all over again. This isn’t some abstract but actual on the trail observation. There is a reason springs are made from steel or titanium, and not aluminium. Lateral flex could certainly be noticed on hardtails with vee’s or canti’s, when honking out of the saddle you could hear the brakes rubbing on the rims.

I can remember years ago trying a trek carbon road frame which felt like it had been made from a piece of wood compared to the 853 frame which was my normal ride.

the late Sheldon Brown does (as usual) an excellent explanation – but it really pre-dates the success of carbon fibre (US fiber) technology

http://www.sheldonbrown.com/frame-materials.html#stiffness

is’nt stiffness the opposite to sloppy design?

There is a reason springs are made from steel or titanium, and not aluminium.

What reason is that then?

You don’t think there’s some chance the rather different geometry of the M800 and Handjob might have had something to do with the perceived difference in comfort? Not that I trust actual on the trail observations, given how inaccurate and variable from day to day the human body is as a measuring instrument, and how susceptible it is to the placebo effect.

By that I presume you mean the fork flex TJ mentioned earlier?

Nope. Top tube/seat tube deflection too.

There is a reason springs are made from steel or titanium, and not aluminium.

Fatigue limit is not the same as modulus of elasticity.

Pretty much everyone on this thread would do well to read these articles[/url] by Scot Nicol. Written nearly 15 years ago but more than relevant to the above discussion.

I’ve got three Whyte 19’s, (an original AN6 alloy, a race with carbon seat stays and a Ti) they all have the same forks stem bars and to be totally honest I’m not sure if I can tell if one is stiffer than the other, but I love riding all three of them so what does it matter?

Nope. Top tube/seat tube deflection too.

Top/down tube, shirley?

The magnitude of which is what though? Sure it has more effect than rear triangle flex, but we’re still talking tiny deflections compared to the other flex in the whole system (like fork flex!)

Dibbs – you heathen unbeliever!

since for the rear triangle to flex vertically you have to compress the stays along their length

No, not really – they can bow outwards or inwards. If you have S shaped ones as many bikes do, they can compress like springs. Only a tiny bit, obviously.

If you have S shaped ones as many bikes do, they can compress like springs. Only a tiny bit, obviously.

Except that springs don’t compress like that. Coil springs are compact torsion bars.

Top/down tube, shirley?

No – the loads encountered by the front “triangle” deflect it in the manner on a cantilevered beam. Triangles are good at supporting point load (like in a rear triangle), but a front triangle doesn’t have loads applied like that.

I am enjoying this thread.

There aint half some pish being spouted on it though.

Cracking troll BTW.

I am looking forwards to warming my hands on my ti frame at the bottom of the next downhill, with the heat that built up in it due to hysterical forum postings.

Anyone mentioned resonance yet?

Or the adverse effects of a misplaced gusset?

No – the loads encountered by the front “triangle” deflect it in the manner on a cantilevered beam. Triangles are good at supporting point load (like in a rear triangle), but a front triangle doesn’t have loads applied like that.

Not quite sure what you mean by that (I understand cantilevered – did structures at uni – just not why front triangle is like one). Which tubes are doing what in terms of bending or longitudinal forces? Surely the seat tube which you suggest is significant is simply in compression? Really interested in a proper explanation if you can give one, as I’m quite happy to admit that it all gets more complex in the front triangle than any of the simple structural analysis I’ve ever done, which has always assumed pinned joints and an effectively zero length head tube.

No, not really – they can bow outwards or inwards. If you have S shaped ones as many bikes do, they can compress like springs. Only a tiny bit, obviously.

Except that the seatstay load path on most bikes is within the tube (which is pretty much a given for a straight tube with normal end loading), hence they won’t be bowing at all. I accept there is maybe a little more flex with s-bend stays, though it’s still pretty insignificant – meanwhile the “comfort” of steel/ti is equally attributed to bikes with straight stays.

My mate used to have a very cheap racer (back in the day before we started calling them road bikes) that was so flexy you could very easily make the back tire rub on the chain stays when riding it.

Dunno what it was made of though, tin maybe.

Ah, the misplaced gusset;

’tis a terrible affliction that manifests itself in spurious ramblings and verbal excreta. Commonly known as “getting your knickers in a twist”, about nothing of import.

Stiffness can be both good and bad, dependant upon what component we are talking, and in what context.

An example is in motor sport; a few years ago it was accepted by the “experts” that a stiff motorcycle frame was the answer to all handling problems; checkout the Sheene and Roberts era GP bikes and you’ll see they had massive power through flexy frames. However, as suggested above, a certain amount of flex, in the right direction, actually helps the handling. Quite a long time ago I used to be involved in Karting. A friend of mine entered the schoolboy championships with a chassis made of wood and won nearly every race. The other entrants had much stiffer steel frames, which meant on the corners they were lifting wheels and losing grip, whereas my mate’s Kart, because of the flex in the wood, easily kept all it’s wheels in contact with the ground. He could literally drive around the outside of people on certain corners.

I had a Cotic Soda, that when I was sat on it stationary, I could bend and flex and twist the whole front triangle. However, when riding i couldn’t detect any of that – I was focusing more on the ride than the frame flex.

Except that springs don’t compress like that.

Coil springs aren’t, no. There’s more than one kind of spring tho.

Except that the seatstay load path on most bikes is within the tube (which is pretty much a given for a straight tube with normal end loading), hence they won’t be bowing at all.

Hmm, not sure about that. If you build a bike with massive chunky seat stays, you’d look at it and think ‘that looks harsh riding’ and you’d probably be right.

Old Pace square tube frames had plain square tubes as seat and chain stays. Later models had the same tubes and material, but the faces of the tubes were milled out. Result – a much more comfortable frame. Why? something’s flexing somewhere. My money’s on the chain and seat stays 🙂

My money’s on placebo effect. Nice to see you’re using “it looks uncomfortable so it must be” now.

My money’s on placebo effect.

🙄

Nice to see you’re using “it looks uncomfortable so it must be” now.

Cept that’s not remotely what I said. I was drawing a link between engineering and people’s experiences. If you see something thick and heavy looking chances are it won’t be supple and light. Your own experience of the world tells you that.

Is this attack Molgrips day or what? Why all the aggro?

If you see something thick and heavy looking chances are it won’t be supple and light. Your own experience of the world tells you that.

The whole point is that it doesn’t make any significant difference how chunky you make the stays since the flex there is already negligible. Therefore when the average person looks at a chunky set of seat stays and thinks that will make the bike “harsh riding” relative to another one with much more slender stays they’d be wrong!

There’s so much of this that isn’t “common sense”, that appealing to that is ludicrous.

The whole point is that it doesn’t make any significant difference how chunky you make the stays since the flex there is already negligible.

I just disagree! Large diameter tubes = stiff tubes, that’s surely clear?

Can someone do FEA on some models of frames so we can see the various modes of vibration caused by different types of trail noise?

but the faces of the tubes were milled out. Result – a much more comfortable frame.

I see 2 factors:
1) we are all highly suggestible and willing to feel/see/hear things which are not really there
2) it doesn’t matter how flexy the frame is, it will still not absorb any of the vibration – the best example being a spring, which undergoes massive flexion, yet they are never used without damping because they just store and release the energy

F*ck off barnes – you didn’t ride the bike, you have no idea. Don’t tell me I’m a highly suggestible imbecile that can’t tell sh*t from shinola without the attached marketing material.

Springs are a shite example because they’re nothing like bike frames. If trail vibrations set up say a transverse oscillation mode in your seat stay, where’s all the kinetic energy of the trail impacts going? left and right, that’s where, not up and down into the seatpost and your arse. Why don’t you try this experiement: get a nice green flexy stick and fix it to the front of your car. Then push the stick repeatedly. Does the car move? No. Does the stick flex? Yes. So the vibrations of your hand moving are being absorbed by the stick, cos the stick is flexy.

Why do you think some frames are harsher than others then? Or are we all imagining that too?

Don’t tell me I’m a highly suggestible imbecile

being clever may actually make one more susceptible to suggestion, and I said “we are all”

get a nice green flexy stick and fix it to the front of your car.

car, wot car ?

So the vibrations of your hand moving are being absorbed by the stick, cos the stick is flexy.

I this has to count as one of the worst experiments not involving death ever and for the life of me I cannot understand what it’s supposed to demonstrate 😐

Why do you think some frames are harsher than others then?

I don’t know that I do think that – there are so many different factors it’s hard to pin down a single effect, particularly if you’re measuring with a flexible and possibly delusional doofus made of meat :o)

So, the stick is absorbing energy of you pushing it?

We have finally debunked Newton’s 3rd law.

I think you’re ignoring the fact that the force exerted is simply not enought to overcome the reciprocal force of the car, and that the stick then returns to it’s original state when you release your force.

PS. I’m really enjoying this troll.

Calm down dear, it’s only the internet! You really should try reading the posts you’re replying to – Simon’s not being personal, just pointing out that everybody is susceptible to suggestion. Meanwhile if you read mine properly, you’d notice that I point out it really doesn’t make any difference if you make the tubes stiffer, since they are already stiff enough that any flex is totally overwhelmed by flex elsewhere. That and the diameter is irrelevant to the stiffness of a tube in compression – the only important quantity is the total amount of metal used.

You don’t need to do FEA – basic structural analysis is enough to show that any deflections in the frame are tiny.

Why do you think some frames are harsher than others then? Or are we all imagining that too?

Yes, you’ve finally got it!

I’m really enjoying this troll.

Ah, but who’s trolling, and given your serious reply, are you feeding him?

Oh, but 😆 at the stick experiment, now I’ve worked out what he’s on about. Try attaching your stick to a balloon, molgrips, and see how much energy it absorbs then!

Later models had the same tubes and material, but the faces of the tubes were milled out.

It was actually a constant section extrusion in the rear stays. It wasn’t milled out.

Hey brant – any comments on my serious question about why you’re considering the main triangle front end as a cantilevered beam, and why the seat tube stiffness is important?

Try attaching your stick to a balloon

if you haven’t got a balloon will a condom do instead ?

PS my reference to ‘doofus’ with not intended as in insult, except to myself 🙂
PPS I’m still working through the article[/url] posted above, which is quite informative.